The present invention relates particularly to a suitable film as the polarizing plate protective film of a liquid crystal display unit.
A liquid crystal display unit (LCD) can be operated at low voltage as well as low electric power consumption, can be directly connected to an IC circuit, and can be specifically produced in a shape having less thickness. Accordingly, the liquid crystal display unit has been widely employed as a display unit for word processors, personal computers, and the like.
Such an LCD is basically constructed in such a manner that, for example, both sides of a liquid crystal cell are provided with polarizing plates.
The polarizing plate transmits light having a polarization plain in the definite direction. Accordingly, the LCD plays an important role to visualize the variation of orientation of liquid crystal due to an electric field. Namely, the performance of the LCD largely depends on that of the polarizing plate.
FIG. 1 shows a common structure of the polarizing plate. In FIG. 1, reference numeral 1 is a polarizer, and both sides of said polarizer 1 are laminated with polarizing plate protective films 2. The LCD is constructed by laminating a liquid crystal cell with the polarizing plate having such a structure.
Polarizer 1 is prepared in such a manner that iodine and the like are adsorbed onto a polymeric film and the resulting film is stretched. Specifically, after a solution called H ink containing a dichroic substance (iodine) is wet-adsorbed onto a polyvinyl alcohol film, the dichroic substance is allowed to orient in one direction by uniaxially stretching the resulting film.
Employed as the polarizing plate protective film 2 are cellulose resins, especially cellulose triacetate.
Said polarizing plate protective film 2 is produced employing apparatuses of a drum casting system shown in FIG. 2 and a belt casting system shown in FIG. 3.
In the drum casting apparatus, casting die 12 is provided adjacent to casting drum 11 and tenter drying section 14 via peeling roll 13, roll drying section 15 and winding roll 16 are provided. After a dope from the casting die is cast onto the casting drum, the resulting film is peeled from the casting drum 11, employing the peeling roll 13, is dried in a first, second, third, and fourth drying zones of the tenter drying section 14, completely dried in the roll drying section 15, and wound onto the winding roll 16.
In the belt casting apparatus shown in FIG. 3, casting die 22 is provided adjacent to casting belt 21, and roll drying section 24 via peeling roll 23 and winding roll 25 are provided. After a dope from the casting die 22 is cast onto the casting belt 21, the resulting film is peeled employing the peeling roll 23, completely dried in the roll drying section 24, and then wound by the winding roll 25. In this casting belt, the cast dope is dried at a high rate. As a result, the tenter drying section employed in the drum casting system is not required.
At present, the thickness of the polarizing plate protective film 2 is at least 80 xcexcm, which is relatively thick.
Recently, from the viewpoint of optical properties such as transmittance and the like, a decrease in the thickness of the polarizing plate protective film has been demanded.
However, when the polarizing plate protective film having a thickness of below 80 xcexcm is produced, it has been found that a number of wrinkles are formed in a film which is wound by the winding rolls 16 and 25 to markedly decrease the production yield.
An object of the present invention is to provide a suitable film employed for a liquid crystal display member, which results in minimal wrinkles which are likely to be formed when said film has a thickness of below 80 xcexcm, increases the production yield, enhances transparency as well as retardation due to a decrease in the thickness.
Furthermore, a decrease in the thickness of a visibility angle enlarging film provided in the entire surface of a liquid crystal display apparatus and the like has been demanded, and said film has had the same problems as those of the polarizing plate protective film.
As the results of investigation performed by the present inventors, it has been discovered that as variation in film thickness increases, more wrinkles are formed, while as variation of the same decreases, less wrinkles are formed.
Further, as the results of detailed investigation on the ratio of wrinkle formation as well as variation in film thickness, it has been discovered that at the film thickness of no more than 60 xcexcm, when variation in film thickness is within xc2x13.0 percent of the standard thickness (average film thickness) of said film, is preferably xc2x12.9 percent, and is more preferably xc2x12.8 percent, wrinkles, which are not permitted as the polarizing plate protective film, are not formed.
Further, it has been discovered that when a film having a thickness of no more than 60 xcexcm is comprised of a cellulose ester film, frequency of wrinkle formation largely depends on the degree of average substitution. The average degree of substitution as described herein is defined based on the average number of hydroxyl groups among three of them in a unit composing cellulose, substituted with other substituents.
As the results of detailed investigation on the frequency of wrinkle formation as well as the average degree of substitution (the average number in entire cellulose of hydroxyl groups of three of them in a unit of C6H10O5 composing said cellulose substituted with other substituents), it has been discovered that at the film thickness of no more than 60 xcexcm, if the average degree of substitution is preferably between 2.88 and 3.00, is more preferably between 2.90 and 3.00, and is most preferably between 2.91 and 3.00, wrinkles which are not permitted in the polarizing plate protective film, markedly decrease.
The present invention has been achieved based on the aforementioned discoveries, and the aforementioned problems are solved employing a film for a liquid crystal display unit, which has a film thickness of 20 to 60 xcexcm and a variation in film thickness within xc2x13.0 percent of the standard film thickness. The variation in film thickness is preferably within xc2x12.9 percent of the standard film thickness, and is more preferably within xc2x12.8 percent.
When the variation in film thickness is controlled within xc2x13.0 percent of the standard film thickness, in the case of a thin film in the range of 20 to 60 xcexcm, especially in the range of 25 to 55 xcexcm, and further in the range of 30 to 55 xcexcm, wrinkles are not substantially formed. Namely, wrinkles, which are not permitted in the film for a liquid crystal display unit, especially a polarizing plate protective film, are not substantially formed. For example, when a 5000 m long film is wound, wrinkles, which are not permitted in the polarizing plate protective film, are not formed. As a result, the production yield increases.
Further, when the film for a liquid crystal display unit is comprised of cellulose ester film, adjusting the average degree of substitution to the range of 2.88 to 3.00 effectively minimizes wrinkles.
Namely, when a film comprised of cellulose ester having an average degree of substitution of 2.88 to 3.00 is wound in a roll, the formation of wrinkles is markedly minimized as the film thickness decreases from the range of 20 to 60 xcexcm via the range of 25 to 55 xcexcm, to the range of 30 to 55 xcexcm. Namely, when the average degree of substitution of cellulose ester is at least 2.88, is preferably at least 2.90, and is more preferably at least 2.91, the film comprised of said cellulose ester is not subjected to wrinkling even using the thin film having a thickness of no more than 60 xcexcm., that is, wrinkles which are not permitted in the polarizing plate protective film are not substantially formed. For example, when a 5000 m long film was wound, wrinkles, which were not permitted in the polarizing plate protective film, were not found. As a result, the high production yield was obtained.
The cellulose ester employed in the present invention preferably has a composition ratio of wood pulp cellulose/cotton linter cellulose=60/40 to 0/100 (in terms of weight ratio). When the polarizing plate protective film is comprised of the cellulose ester having such a ratio, a high quality polarizing plate protective film, having minimal wrinkles, is obtained.
The polarizing plate film having a thickness of no more than 60 xcexcm is preferred because higher transparency as well as less phase difference is obtained.
Further, the polarizing plate protective film having a thickness of no more than 55 xcexcm is more preferred. With the lower limit of the thickness, the thinner the film, the more it is preferred, if the resulting film exhibits sufficient mechanical strength. However, at present, from the viewpoint of the mechanical strength, the lower limit of the thickness is 20 xcexcm, is preferably at least 25 xcexcm, and is more preferably at least 30 xcexcm.
Namely, when conditions to achieve the thickness of 20 to 60 xcexcm as well as the variation in the film thickness within xc2x13.0 percent of the standard film thickness are satisfied, and further, the condition to achieve the average degree of substitution of 2.00 to 3.00 are simultaneously satisfied, first, problems with optical properties (transparency and phase difference properties) as well as wrinkle were simultaneously overcome and a high quality polarizing plate protective film was obtained.
The aforementioned film preferably has a tear strength of at least 7 g. The tear strength of at least 7 g is preferred, considering the mechanical strength as the polarizing plate protective film, as well as the winding operation during its production, which is carried at a high speed while applying tension as high as possible to the film so that wrinkles are not formed. The upper limit is not definite. However, when the element materials, which are now employed for the polarizing plate protective film, are taken into account, the specific tear strength is no more than 50 g.
A film is preferably comprised particularly of lower fatty acid esters of cellulose. Further, the film preferably comprises plasticizers in an amount of 1 to 30 percent by weight. Still further, the film preferably comprises an UV absorber in an amount of 0.01 to 5 percent by weight.
The film constructed, as described above, exhibits a haze of no more than 0.5 percent (preferably no more than 0.4 percent). Further, the film has a retardation value of no more than 10 nm (preferably no more than 8 nm, and more preferably no more than 4 nm). The film, having a haze of no more than 0.5 percent, preferably no more than 0.4 percent as well as a retardation value of no more than 10 nm, preferably no more than 8, exhibits sufficient performance as the polarizing plate protective film.
A film employed in a liquid crystal display member wherein film thickness is between 20 and 60 xcexcm and variation in the film thickness is within xc2x13.0 percent of the standard film thickness.
A film employed in a liquid crystal display member wherein said film comprises cellulose ester having a ratio of wood pulp cellulose/cotton linter cellulose in the range of 40/60 to 0/100 (in weight ratio), and the thickness of said film is between 20 and 60 xcexcm and the variation in the film thickness is within 3.0 percent of the standard film thickness.
A film employed in a liquid crystal display member wherein variation in the thickness of said film, which is wound over a length of 1500 m, is within xc2x13.0 percent of the standard film thickness.
A film employed in a liquid crystal display member wherein the thickness of said film is between 20 and 60 xcexcm, and the variation in the thickness of said film, which is wound over a length of 1500 m, is within xc2x13.0 percent of the standard film thickness.
A cellulose ester film wherein a film thickness is 20 to 60 xcexcm and an average degree of substitution is 2.00 to 3.00.
A cellulose ester film wherein a film thickness is 20 to 60 xcexcm, and an average degree of substitution of the said film is 2.88 to 3.00, when said film having said thickness is wound in a roll over a length of at least 1000 m after saponification.
A production method of a cellulose ester film wherein a dope composition prepared by dissolving in a solvent composition materials for said film, which comprises cellulose ester having an average degree of substitution of 2.00 to 3.00, is cast into a film having a thickness of 20 to 60 xcexcm employing a solution casting method.
A polarizing plate wherein said plate is provided with said cellulose ester film and a polarizer in which said cellulose ester film is overlay on said polarizer.
A polarizing plate wherein said plate is provided with said cellulose ester film and a polarizer in which said cellulose ester film is overlay on both surfaces of said polarizer.